Gangliosides in cell recognition and membrane protein regulation

Abstract

Gangliosides, sialic acid-bearing glycosphingolipids, are expressed on all vertebrate cells, and are the major glycans on nerve cells. They are anchored to the plasma membrane through their ceramide lipids with their varied glycans extending into the extracellular space. Through sugar-specific interactions with glycan-binding proteins on apposing cells, gangliosides function as receptors in cell-cell recognition, regulating natural killer cell cytotoxicity via Siglec-7, myelin-axon interactions via Siglec-4 (myelin-associated glycoprotein), and inflammation via E-selectin. Gangliosides also interact laterally in their own membranes, regulating the responsiveness of signaling proteins including the insulin, epidermal growth factor, and vascular endothelial growth factor receptors. In these ways, gangliosides act as regulatory elements in the immune system, in the nervous system, in metabolic regulation, and in cancer progression.

Figure 1

Selected ganglioside structures and biosynthesis. The structure of GT1b is shown above the biosynthetic pathways of brain gangliosides. Three glycolipid-specific glycosyltransferase genes discussed in the text (St3gal5, St8sia1, and B4galnt1) are indicated. Knockout mice for these genes (alone or in combination) overexpress gangliosides upstream of the block, such that the total ganglioside concentration remains relatively constant. For example, the quantitatively minor brain gangliosides GM1b and GD1α (shown as faded) become dominant in St3gal5-null mice.

Figure 2

GT1b analog bound to Siglec-7. (A) The V-set domain of Siglec-7 was co-crystallized with a GT1b analog (2-(trimethylsilyl)ethyl glycoside of GT1b oligosaccharide). A stereo image of the glycan binding site surface is shown. The network of potential hydrogen bonds is shown as black-dashed lines with stably associated water molecules as orange spheres. (B) and (C) The electrostatic surface potential of the apo and liganded forms, respectively; basic regions are colored blue, and acidic regions are red. The C-C' loop in the unliganded structure lies in an extended conformation with a concave shape (dotted line in B). Upon the binding of GT1b, a major shift occurs. In the liganded structure the C-C' loop presents a hydrophobic convex shelf-like surface (dotted line in C). In the C'-D loop region, Trp-85, which lies across the surface, flips out, becoming fully solvent exposed and causing the C'-D loop to adopt a helical conformation. As a result a large hydrophobic patch opens up on this side of the binding site (C). Adapted from [8], with permission.

Figure 3

Proposed mechanism for a shift of the insulin receptor (IR) from caveolae leading to insulin resistance. IR may be constitutively resident in caveolae via binding to the scaffolding domain of caveolin-1. Binding of IR and caveolin-1 is necessary for insulin metabolic signaling. The localization of IR in the caveolae is interrupted by elevated levels of the endogenous ganglioside GM3 during a state of insulin resistance (e.g. induced by TNFα). IRS-1, insulin receptor subtrate 1; PI3K, phosphoinositide 3-kinase; GLUT4, insulin-regulated glucose transporter. Adapted from [38], with permission.

Figure 4

(Upper panels) Imaging mass spectrometry (50 µm raster step size) was used to gain an overview of disialoganglioside (GD1) distribution in different regions of a sagittal section in the mouse. Differential distributions of ions representing gangliosides with 18-carbon (green) and 20-carbon (red) sphingosines are apparent. (Lower panels) Imaging mass spectrometry (15 µm raster step size) to study the detailed distribution of gangliosides in the hippocampus. Adapted from [56], with permission.